1. Field of the Invention
The present invention relates to a branching filter for separating a plurality of signals of mutually different frequencies from each other.
2. Description of the Related Art
Mobile communication systems conforming to the Long Term Evolution (LTE) standard have become practically used in recent years, and further, practical use of mobile communication systems conforming to the LTE-Advanced standard, which is an evolution of the LTE standard, is under study. Carrier Aggregation (CA) is one of the key technologies of the LTE-Advanced standard. CA uses multiple carriers called component carriers simultaneously to enable wideband transmission.
A mobile communication apparatus operable under CA uses multiple frequency bands simultaneously. Accordingly, such a mobile communication apparatus requires a branching filter capable of separating a plurality of signals in a plurality of frequency bands from each other simultaneously.
Typically, a branching circuit for separating two signals in mutually different frequency bands from each other includes a common port, a first signal port, a second signal port, a first filter provided in a first signal path leading from the common port to the first signal port, and a second filter provided in a second signal path leading from the common port to the second signal port.
The first and second filters can be LC filters formed using inductors and capacitors, or acoustic wave filters formed using acoustic wave resonators. The acoustic wave resonators are resonators formed using acoustic wave elements. The acoustic wave elements are elements that use acoustic waves. The acoustic wave elements include surface acoustic wave elements using surface acoustic waves, and bulk acoustic wave elements using bulk acoustic waves.
JP-H04-196829A discloses a branching circuit including an antenna terminal, a reception signal output terminal, a transmission signal input terminal, a transmission system between the antenna terminal and the reception signal output terminal, and a reception system between the antenna terminal and the reception signal output terminal. In this branching filter, each of the transmission system and the reception system includes a dielectric resonator and a surface acoustic wave filter connected in a cascade arrangement in this order from the antenna side.
U.S. Patent Application Publication No. 2002/0030555 A1 discloses a branching filter (duplexer) including first to third terminals, a branch circuit connected to the first terminal, a receiving-side LC parallel resonator-type filter, a surface acoustic wave filter, and a transmitting-side LC parallel resonator-type filter. The receiving-side LC parallel resonator-type filter and the surface acoustic wave filter are provided between the branch circuit and the second terminal in this order from the branch-circuit side. The transmitting-side LC parallel resonator-type filter is provided between the branch circuit and the third terminal.
U.S. Pat. No. 8,384,496 B2 discloses a band-stop filter including a first terminal, a second terminal, a first filter branch and a second filter branch connected in parallel between the first terminal and the second terminal, a bandpass filter arranged in the first filter branch, a high-pass filter arranged in the second filter branch, and a surface acoustic wave resonator provided in the path connecting the second filter branch to the ground.
Mobile communication apparatuses require miniaturization of components used therein. Mobile communication apparatuses may further require a branching filter for separating two signals in two mutually relatively close frequency bands from each other. Such a branching filter requires a filter having an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency.
For LC filters, typically, attempting to achieve an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency results in increase in size of the filters.
On the other hand, while acoustic wave filters are suitable to achieve an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency, they are not suitable to provide a wide passband.
Thus, it has conventionally been difficult to provide a branching filter that is suitable to separate two signals in two mutually relatively close frequency bands from each other and is miniaturizable.